To the Editor:— Airway management is a major responsibility for the anesthesia personnel. Patients with obvious signs suggestive of difficult direct laryngoscopy and intubation (i.e. , orofacial abnormalities, short thick neck, protruding teeth, high arches palate, and others) are usually treated with extreme caution. However, some patients who appear normal on conventional examination may still present an unanticipated difficult airway. It is all the more alarming in morbidly obese patients who have a higher-than-usual incidence of difficult mask ventilation and precipitous oxygen desaturation. 1Here I present a case of unanticipated difficult intubation in a morbidly obese patient in whom the difficulty in performing direct laryngoscopy and intubation was a direct result of altered neck extension and mouth opening resulting from a change in the position of the patient from the sitting to the supine position.
A 35-yr-old African-American man (weight, 190 kg; height, 178 cm) presented for gastric bypass surgery under general anesthesia. He had no significant medical history. His preoperative airway examination revealed relatively large face, a thick neck, adequate mouth opening (> 4 cm), and intact upper and lower dentition with no evidence of overbite. He was assigned an airway Mallampati score of II. He had a full range of active neck flexion and extension. All examination was performed while the patient was in the sitting position. A doughnut-shaped pillow (10 cm thick) was placed under his head. Anesthesia was induced with thiopental sodium, and muscle relaxation for intubation was achieved with succinylcholine. Soon after fasciculation had passed, we faced significant difficulty extending the neck and opening his mouth. A slight extension at the neck allowed us to open the mouth enough to introduce the Macintosh No. 4 blade. Further attempts at neck extension were unsuccessful. The headboard of the operating table was lowered, with no improvement in neck extension. A roll of blanket was placed under the shoulder, which seemed to worsen the overall extension at the neck. At that point, the head was supported, and the intubation was accomplished successfully on the second attempt with only the posterior tip of the arytenoids visualized.
At the end of the surgical procedure, the patient was extubated while awake. On the third postoperative day, I reexamined the airway with the patient in the sitting position, with no change in the findings. However, when he was asked to lie on the bed and open his mouth, he could open his mouth only while actively extending his head. On closer examination of his neck, it was clear that his skin and tissues at the back of his neck and, especially, the upper shoulder were thick and excessive. Interestingly, the full range of motion at the neck and adequacy of mouth opening was well persevered on asking the patient to perform these maneuvers while in the lateral position.
Various investigators have found that a Mallampati score of III or IV was a risk factor for difficult intubation. 1However, the sensitivity, specificity, and negative predictive value of the Mallampati score are poor. In addition, investigators have found no correlation between body mass index and difficult intubation in obese patients. 1
Adequate mouth opening and neck mobility are the two most important variables defining ease of direct laryngoscopy and intubation. 2It is generally understood that these variable are well persevered in the supine position. Tham et al. , 3in their study evaluating the effect of posture on the Mallampati class, concluded that posture has no effect on the class assigned in the Mallampati test performed in the sitting position. It is well known that both Mallampati and the later suggested Samsoon and Young modification indicate that these assessments should be performed with the patient in the sitting position. 4
In the current case, a closer look revealed that as the patient was asked to lie down on the operating table, the tissues on the posterior upper aspect of the shoulder were squeezed in the direction of the back of the neck, thereby crowding the area behind the neck. This tissue at the back of the neck in the supine position seemed to restrict the extension at the neck whenever the patient was placed in the supine position with his shoulder positioned over the relatively hard operating room mattress. Interestingly, the patient could extend his neck to a greater extent when he was allowed to support his upper body on the elbows while in the supine position. Placing a roll of towels under the neck seemed to further limit the extension at the neck by facilitating the redistribution of the tissues to behind the neck.
Yet another interesting finding in this patient was our inability to open the mouth adequately when we placed him supine on the operating room table with his head resting on the doughnut-shaped pillow. The cause of this is unclear, although the patient could demonstrate adequate mouth opening in supine position for as long as he could extend his neck simultaneously. Theoretically, it is possible for the tissues in the neck, especially those in the posterior region of the neck (behind the angle of the mandible), to become crowded enough. Perhaps it is this change in the range of passive mobility at the atlantooccipital joint coupled with restricted jaw mobility in the anesthetized paralyzed patient placed in supine position, which may be one of the causes of unanticipated difficulty in intubation. Because during passive jaw opening the angle of the mandible moves backward, the ease of doing that depends in part on the softness of the tissues that it displaces. In my personal experience, I have encountered this problem most often in patients with thick muscular necks with prominent sternocleidomastoids.
The difficult tracheal intubation is more common among obese than nonobese patients. Unfortunately, among the classic risk factors for difficult intubation, only a Mallampati score of III or IV has been identified as a risk factor in obese patients. It includes assessment of range of extension of the cervical spine, mobility at the temporomandibular joint, maximum mouth opening, and tongue size. Here I have identified two such factors, which may contribute to the increased incident of difficult laryngoscopy and intubation in a subset of obese patients. Consequently, I recommend that all obese patients and patients with thick, short necks be reevaluated while in the supine position for adequacy of range of motion and mouth opening to avoid unanticipated difficult airway on induction.